Viruses
Structure of Viruses :- Viruses are infectious agents. They are much smaller than the bacteria. Their approximate size ranges from 20 to 300 nm. Viruses are
incapable of independent growth in artificial media. They can grow only in
animal or plant cells or in microorganisms. Hence they are referred to as
obligate intracellular parasites. They reproduce in these cells by replication.
Replication is a process in which many copies or replicas of the viral
component are assembled and made to represent the progeny. They lack
metabolic machinery of their own to generate energy or to synthesize
proteins. They depend on the host cells to carry out these vital functions. The
bacterial viruses are called bacteriophages. These viruses infect the
bacteria and multiply inside the bacterial body and cause the lysis of bacteria
(lytic cycle) or integrate themselves with the bacterial genome (lysogeny).
Structure of Viruses:
Animal and plant viruses are composed of a central core of nucleic
acids surrounded by a protein covering called capsid. The capsid is made up
of units called capsomeres. Viruses exhibit a characteristic symmetry,
1. Spherical viruses are isohedral, 2. Rod shaped viruses are helical in
symmetry. Certain group of viruses are complex in symmetry.
Some animal viruses, in addition to the nucleocapsid structure
contain an outer membrane like structure called the envelope. The envelope
is made up of lipoproteins. The envelope conceals the symmetry of viruses.
Virions with envelopes are sensitive to lipid solvents such as ether and
chloroform. On the other hand, the naked virions are not affected by the
lipid solvents.
Isohedral viruses such as adeno viruses, SV15, polio viruses and
blue tongued viruses are spherical in shape and their surface is a lattice with
identical triangular units.
Helical surface symmetry and structure are characteristic of tobacco
mosaic virus(TMV) and animal viruses that cause diseases such as measles,
mumps, influenza and rabies. In these, the nucleo capsid is a flexible
structure packed within a fringed lipoprotein envelope. The fringes are made
of glycoproteins. In TMV the nucleic acid core is covered by a capsid
consisting of closely packed capsomeres arranged in a helix.
Complex or uncertain symmetry is seen in Pox viruses, T-bacteriophages,.
These have different proteins and lipoproteins.
Related Topics in Zoology:
Bio Zoology All Important Topics
- Microbiology Introduction and History of Medical Microbiology
- Pasteur, Koch, Lister
- Structure of Viruses
- Viral genetics
- Virus Culture
- Viral Diseases
- Bacteria Structure Culture
- Bacterial Genetics
- Bacterial Diseases
- Protozoan microbiology
- Pathogenecity of Microorganisms
- Antimicrobial Resistance
- Antibiotics and Chemotherapy
- AIDS – HIV